Passive infrared intrusion detection systems are known for sensing the presence of an intruder in a protected area and providing an output signal representative of intruder detection. The performance of such systems is often specified for a particular range; that is, the distance from an intruder to the detector. The performance of the system outside of the specified range can become significantly diminished and thus the ability to reliably detect intruder presence outside of the specified range is reduced. In a single focal length system, an intruder travelling at uniform velocity will be in the field of view longer at further distances from the detector than at shorter distances. The detection sensitivity is therefore variable with the distance of an intruder from the detector. It would be useful to optimize system performance over widely different ranges to provide reliable intruder detection anywhere within a protected space.
Examples of passive infrared intrusion detection systems are shown in U.S. Pat. Nos. 3,036,219; 3,524,180; 3,631,434; 3,703,718; and 3,886,360. In U.S. Pat. No. 3,886,360 an infrared intrusion detection system is shown in which an array of spherical mirror segments is employed to provide greater intensity of radiation received from more distant objects. In one disclosed embodiment, the mirror segments are of the same focal length with the sensing element being disposed asymmetrically to receive more radiation from those segments collecting radiation from the further objects. In a second disclosed embodiment, the spherical mirrors are of different focal lengths, each spaced its focal distance from the detector to provide greater collection of radiation from the more distant objects.